Problem with storage?Support

I own a GarminAsus Nuvifone A50 and I dont know how much app space that I have. When I go to Settings and Storage it says 400 free app storage of 500 something. But I know that my apps arent stored there. I have an app named History Eraser that I use to clear Cache and delete market history. Theres an option that enables you to see memory and its somewhat 155. I currently havr 90 free space. Heres the problem. Even if I uninstall an app, it doesnt free any of those 150 MBs... why is that? And how can I use the app storage? I can take a snapshot if needed.